Ink jet printers, in particular drop-on-demand (DOD) or impulse printers having ink jet print heads with acoustic drivers to accomplish ink drop formation, are well known in the art. For example, ink jet print head designs, in which ink is ejected from the print head in a direction perpendicular to the plane of one or more ink pressure chambers, are disclosed in U.S. Pat. No. 4,266,232 issued to Juliana, Jr. et al.; U.S. Pat. No. 4,312,010 issued to Doring; U.S. Pat. No. 3,747,120 issued to Stemme; U.S. Pat. No. 4,599,628 issued to Doring et al.; U.S. Pat. No. 4,680,595 issued to Cruz-Uribe et al.; and U.S. Pat. No. 4,460,906 issued to Kanayama. Print head designs that eject ink in a direction parallel to the plane of one or more ink pressure chambers are disclosed, for example, in U.S. Pat. No. 4,216,477 issued to Matsuda et al.; U.S. Pat. No. 4,525,728 issued to Koto; U.S. Pat. No. 4,584,590 issued to Fishbeck et al.; U.S. Pat. No. 4,435,721 issued to Tsuzuki; U.S. Pat. No. 4,528,575 issued to Matsuda; U.S. Pat. No. 4,521,788 issued to Kamura and D. E. Patent No. 3,427,850 issued to Yamamuro.
The principle underlying the successful operation of an ink jet print head of this type is the manipulation of pressure within an ink pressure chamber to achieve controlled emission of ink droplets from the chamber through a nozzle orifice or ink drop ejection orifice outlet. In general, a DOD ink jet print head, having an ink pressure chamber coupled to a source of ink and an ink drop ejecting orifice terminating in an ink drop ejection orifice outlet, is operated as set forth below. An acoustic driver expands and contracts the volume of the ink pressure chamber to eject a drop of ink from the orifice outlet. More specifically, the acoustic driver applies a pressure wave to the ink residing within the ink pressure chamber to cause the ink to pass outwardly through the orifice and through the orifice outlet in a controlled manner.
In the prior art, a number of different acoustic drivers have been employed to generate a pressure wave in DOD ink jet print heads. For example, drivers consisting of a pressure transducer formed by bonding a piezoelectric ceramic material to a thin diaphragm have been utilized for this purpose. In response to an applied voltage, the piezoelectric ceramic material deforms and causes the diaphragm to deflect and displace ink in the ink pressure chamber, which displacement results in a pressure pulse or pulse train and, ultimately, the flow of ink through one or more nozzles.
Prior art piezoelectric ceramic drivers have been formed in a variety of shapes, such as circular, polygonal, cylindrical, and annular-cylindrical. In addition, prior art piezoelectric ceramic drivers have been operated in various modes of deflection, such as bending mode, shear mode, and longitudinal mode. Other types of prior art acoustic drivers for generating pressure waves in ink include heater-bubble source drivers (for bubble or thermal ink jet print heads) and electromagnet-solenoid drivers. In general, it is desirable in an ink jet print head to employ a geometry that permits multiple nozzles to be positioned in a densely packed array, with each nozzle being driven by an associated acoustic driver.
Prior art ink jet print heads have experienced difficulty with degradation in printing quality resulting from rectified diffusion. Rectified diffusion occurs after a period of continuous ink jet print head operation as a consequence of the repeated application of pressure pulses, at below ambient pressure, to ink located within the ink pressure chamber. The threshold at which rectified diffusion occurs is dependent upon a number of factors, such as drive pulse shapes and durations, absorbed gas concentrations, temperature, particulate matter (e.g., pigment particles present in the ink) and roughness of the driver surface. The length of time before the onset of printing quality degradation depends on the drop generation rate and, prior to the initiation of repetitive ink jet print head operation, on the amount of air dissolved in the ink, the presence of particulates in the ink, the ink viscosity, the ink density, the diffusivity of air in the ink, and the radii of air bubbles present, if any, in the ink.
As discussed above, ink jet print head designs employing piezoelectric ceramic material deformation/diaphragm deflection operations are characterized by contractions and expansions of ink pressure chamber volume which generate pressure pulses in ink contained in the chamber. Contractions occur rapidly and are preceded and/or followed by rapid expansions of ink pressure chamber volume. During the expansion phase, the pressure in the ink pressure chamber is reduced significantly, increasing the tendency to bubble formation at the chamber surface by air dissolved in the ink. The tendency to bubble formation is highest at nucleation sites on the ink pressure chamber surface where gases may be retained. Nucleation sites include, for example, corners, edges, points, cracks, pits or foreign particle deposits.
In ink characterized by exceeding the condition-dependent rectified diffusion threshold, pressure pulse application results in the formation and/or growth of air bubbles disposed in the ink, rather than oscillation of air bubble size about a mean value. Specifically, more gas is added to the air bubbles during negative pressure (below ambient) applications than is re-absorbed into the surrounding liquid during positive pressure (above ambient) applications. If conditions favorable to air bubble growth persist, large air bubbles will be formed in the ink contained within the pressure chamber.
Gas bubbles in the ink absorb energy supplied to the ink in the ink pressure chamber. As the gas bubbles grow, they absorb more of the energy supplied by the acoustic drivers. When the bubbles attain a large enough size, they absorb so much energy that ink drops cannot be ejected from the nozzles in the ink jet print head at appropriate speeds or volumes through the action of the acoustic driver. If the condition-dependent rectified diffusion threshold is exceeded for a time period exceeding that required for the onset of printing quality degradation, itself a condition-dependent parameter, the prints generated by the ink jet print head will suffer from inexact ink drop ejection.
Rectified diffusion is a recognized problem in the ink jet printing art. As a result, numerous approaches have been employed in an effort to mitigate or alleviate the problem. For example, U.S. Pat. No. 4,947,184 issued to Moynihan discusses coating the entire pressure chamber of an ink jet print head with a smooth, conforming coating layer of material that is wettable by the ink to be contained therewithin. The use of a coating with a surface energy greater than that of the ink is preferred to promote wetting. The smooth coating layer is applied to fill in or otherwise decrease the number of nucleation sites located on pressure chamber surfaces. This prior art coating process is conducted following assembly of the ink jet print head, possibly introducing contamination into the jet, clogging the small passages of the jet, or causing some of the acoustic energy to be absorbed through the addition of such energy-absorbing materials to pressure chamber surfaces.
At the Fifth International Congress on Advances in Non-impact Printing Technologies held in November 1989, Spectra Inc. described a de-aeration process for a DOD ink jet printer. In this technique, the concentration of gas dissolved in the ink, one factor in determining the rectified diffusion and degradation onset time thresholds, is decreased. This decrease in dissolved gas was indicated to alleviate the rectified diffusion problem.